The present invention relates to a control apparatus for induction motors which can individually instruct a torque current component and a magnetizing current component, wherein the equivalent circuit of an induction motor corresponding to one phase is as shown in FIG. 1.
x.sub.1 denotes a primary leakage reactance, r.sub.1 a primary resistance, r.sub.2 ' a secondary resistance (in terms of a primary value), and x.sub.m a magnetization reactance. In this figure, a secondary leakage reactance is omitted. Since, however, it is slight, it exerts no great influence even when neglected.
In a case where, in such circuit, current flowing through the magnetization reactance x.sub.m (it is a current contributive to a magnetic flux, and hereinbelow, it shall be called "magnetizing current" and denoted by I.sub.m) and current flowing through the secondary circuit r.sub.2 ' (it is a current contributive to a torque, and hereinbelow, it shall be called "torque current" and denoted by I.sub.t) are controlled independently of each other, the value of the input voltage (hereinbelow, denoted by V.sub.dc) of frequency conversion means needs to be determined as follows. In case of employing an inverter as the frequency conversion means, the maximum output of the output line voltage of the inverter becomes a waveform shown in FIG. 2, and the maximum value of the fundamental wave becomes ##EQU1## The fundamental wave voltage needs to be greater than the terminal voltage of the induction motor when a voltage drop component in the inverter, a voltage drop component attributed to a line resistance, etc. are taken into account. It needs to satisfy the following equation for the rated values of the currents I.sub.m and I.sub.t, in consideration of the highest rotational frequency at which the induction motor can be operated at a rated torque: ##EQU2## When the voltage V.sub.dc is selected at that minimum value determined by Equation (1) with which an actual primary current I.sub.1 can precisely follow up a primary current command value (hereinbelow, the minimum value shall be called the "rated input voltage of the inverter"), the control apparatus is economical in view of the breakdown voltage characteristics of the main switching elements of the inverter. However, in case of holding I.sub.m constant steadily and controlling I.sub.t in accordance with a required torque, I.sub.m cannot flow as instructed and lowers when V.sub.dc has become lower than the aforementioned rated input voltage.
In many of vector controls, I.sub.m is held constant, and I.sub.t is controlled in accordance with a required torque. However, in a case where the input side voltage of the frequency conversion means has lowered; when the command value of I.sub.m is set at a constant value, it is sometimes the case that I.sub.m as instructed cannot flow. This has sometimes led to an unstable control system, a worsened torque response, and the occurrence of the drawbacks of velocity hunting etc.